Acoustic filter



Oct. 1942- H. K. SCHILLING ACOUSTIC FILTER Filed Oct. 30, 1939 4 Sheets-Sheet 2 I ll $8 '5 V D" INV ENT OR fiar'old If. 5

ATTORNEY-3 Oct. 20, 1942. 5 N 2,299,112

ACOUSTIC FILTER Filed 00t- 30, 1939 4 Sheets-Sheet 4 IHI1 ATTORNEYS Patented Oct. 20, 1942 UNITED STATES PATENT OFFICE Robert 0. Brown, Jr.,

trustee Highland ms, 111., as

Application October 30, 1939, Serial No. 302,007

13 Claims.

This invention is concerned generally with acoustic band stop filters, i. e., devices which, placed in the path of sound, will remove therefrom, or greatly repress, sounds of pitches lyin within a definite frequency range, and readily transmit sounds of pitches lying outside of this range.

More particularly this invention is concerned with acoustic filters which, in addition to possessing band stop characteristics as regards the transmission of sound, also have the property of allowing the ready passage of gases, such as air for ventilation, and the like.

Such filters may have a variety of uses: used purely as acoustic filters they can be placed in front of telephone receivers or microphones to repress sounds of undesirable frequencies. or to isolate, for the purpose of measurement, definite bands of frequencies of an admixture of sounds or noises; used otherwise, they can be placed in ventilating ducts or apertures to repress motor or blower noises, or to prevent, where a ventilating duct communicates with several rooms, noises from being transmitted from one room to another; they may be placed in windows to allow the entrance of air but shut out street noises, or in transoms to shut out hall noises, etc.

One of the objects of this invention is to provide a form 0 acoustic band stop filter of readily variable characteristics, that is to say, a form such that the band width and cutofi. frequencies can be predetermined readily to suit the particular purpose for which the filter is to be used. Another object is to provide an acoustic band stop filter which will produce a maximum repression of frequencies lying within a certain range and yet transmit with a minimum of impedance frequencies outside this range. Further objects are to provide a form of acoustic band stop filter which is readily adaptable to any size or shape of aperture, also one which is simple and inexpensive to manufacture.

Additional objects of the invention are to provide forms of acoustic band stop filters which will be efiective at relatively low frequencies: and also to provide forms which will be efiective on nonplanar sound waves.

Further and other objects will become apparent as the description is read in connection with the accompanying drawings, in which- Fig. 1 is a sectional elevation of a building with an air conditioning and ventilating system associated therewith, all shown more or less diagrammatically, the ventilating system being provided with sound repressing filters;

Fig. 2 is a diagrammatic view of the proportioning of the heights of the filter ducts in the case of a filter of rectangular cross section:

Fig. 3 is a similar view for the case of a filter of circular cross section;

Fig. 4 is a perspective view of one form of band stop acoustic filter in accordance with my invention;

Fig. 5 is a horizontal cross section of the same;

5 Fig. 6 is a graph of the characteristic curve of a filter oi the type shown in Figs. 4 and 5;

Fig. 7 is a graph giving the relation between the cutofl frequencies and the spacing of the filter plates for a filter of the type shown in Figs.

4 and 5;

Fig. 8 is a graph showing the relation between the lower cutoif frequency and the depth of the filter grooves for a filter of the type shown in Figs. 4 and 5:

i 9 is a perspective view of a portion of a modified form of acoustic filter, namely one in which pairs of adjacent slots havebeen united into a single passage:

Fig. 10 shows more or less diagrammatically a 2 form of filter particularly adapted to construction of sheet metal;

Fig. 11 shows a form of cover plate to be used when it is desired to filter out low frequencies: Fig. 12 is a horizontal cross section of a filter adapted to filter out low frequencies;

facture by extrusion or molding;

Fig. 15 is a cross section of a filter plate also for low frequencies adapted to manufacture from sheet metal;

Fig. 16 is a cross section of a filter plate assembly also for low frequencies adapted to be manu tured from a special form of channel;

Fig. 17 is a perspective view of a filter plate particularly effective on nonplanar waves;

Fig. 18 shows a perspective view of an alterna tive form of filter plate for the same purpose;

Fig. 19 shows a perspective view of a form of acoustic band stop filter, particularly efi'ective on nonplanar waves;

Fig. 20 is a front elevational view of an acous- 5 tic filter of variable band width;

Fig. 21 is a side elevational view of the same; Fig. 22 is a perspective view of a special calibrated adjusting nut used in the filter of Figs. 20 and 21;

Fig. 23 is a perspective view of a special resilient washer used in the filter of Figs. 20 and 21;

Fig. 24 is a side elevational view of the stringer bar used in the filter of Figs. 20 and 21;

Fig. 25 is a diagrammatic view of a form of 65 the'invention for delivery of air from a high pressure main to a room; and

Fig. 26 is a diagrammatic view of an improved arrangement for delivery of air from a high pressure main to a room.

The present invention is capable of various forms of construction and practical application. In order to disclose one embodiment of. the invention, it is shown and described in connection with an air conditioning system, but it is understood that this is by way of example only as the invention is capable of use in various other relations.

In the conventional air conditioning systems for dwellings, ofilces, hospitals and similar structures, it is necessary to employ slow speed fans or blowers for circulating the air because the high speed fans or blowers are too noisy and the noises conducted to the rooms by the air ducts from these high speed fans or blowers are very disturbing. The use of slow moving blowers is objectionable because of the inefilciency of the operation of such fans or blowers.

The present invention seeks to remedy this difficulty by the provision of new and improved sound filter means that may be interposed in the air ducts between the blower and the rooms for absorbing not only the obnoxious sound waves or noises generated by the blower but also for filtering out objectionable street noises as well.

Referring now to the drawings, in particular to Fig. 1, the reference numeral 20 designates a dwelling or oifice building having the separate rooms or offices 2| and 22, hallway 23, and the basement 24. The building may comprise anynumber of rooms but only two have been illustrated in order to simplify the description. Located in some convenient place in the basement 26 is a fan or blower 2B driven by an electric motor 26, which blower supplies air to the rooms 2! and 22 by means of the main supply conduit 21, and the branch supply conduits 2! and 28. Air leaves the rooms through the ventilator openings 2| and 32, and returns to the air conditioner 26 by means of the branch return conduits 22 and 34 and the main return header 25. Before entering the conditioner, the return air is mixed with a certain proportion of fresh air, which enters through the fresh air intake 21. The resulting mixture then passes through the air conditioner 38, which may be adapted to heat or cool and humidlfy the air before delivery to the blower 25. Insteadof being returned to the air conditioner, some of the air may escape from the rooms through the window openings 38 and the transom openings 38. Suitable dampers li are provided in the branch condu'its 28, 28, 23, and 24 for regulating the fiow of air to and from the rooms. In addition, louver valves 42 and 42 are provided in the air intake 31 and the return header 2!, respectively, in order to regulate the proportions of fresh and returned air entering the air conditioner 26.

Placed at suitable points in the ventilating system and adapted to fit the various shapes of cross section of the same are acoustic filters according to my invention. In particular. following the blower 25 there is an acoustic filter 44 adapted to fit in the main conduit 21 and designed particularly to filter out fan and motor noises: placed in the conduits 28, 29, 23, and 24 are acoustic filters 4B for the purpose also of filtering out fan and motor noises as well as to prevent noises from being transmitted from one room to another by means of the conduits 21, 22 and 28 or 33, I4 and 25. Placed in the window openings 28 are acoustic filters 46 particularly designed to filter out street noises and permit the escape of air from the building, while in the transom openings 32 are placed acoustic filters 41, particularly designed to filter out hall noises, etc.

The filter consists in general of a number of ducts of similar construction ranged alongside one another, 1. e. in parallel, the exact number being determined in a given case by the conditions to be met: for example, the impedance which the filter as a whole is to offer to the transmitted sound or stream of air. In general, the greater the number of ducts, the greater the effective aperture of the filter and the lower its impedonce. The height of the individual ducts and the total number of ducts will depend also on the shape of the aperture, the area which the filter is to cover, etc. In particular if the aperture in which the filter is placed is of rectangular shape, the filter is preferably of like shape, the individual ducts all having the same height as in Fig. 2; while for other shapes of apertures the height of the individual ducts will preferably conform tothe height of the cross section at any'point, as shown in Fig. 3, which shows a preferred proportioning of the ducts for a circular aperture.

In the form of the construction shown in Figs. 4 and 5, which is by way of example only, the filter comprises the frame 5|, which in the form shown is rectangular in shape and has, what for convenience of description will be termed, the upper and lower frame members I52 and 53 and the end members 54 and 55. Mounted within the frame are a plurality of vertical partitions 58 spaced apart and from the end walls 54 and 55 to form parallel passages or ducts 51 rectangular in cross section, along which the air and sound waves travel from the front toward the back of the filter. Each partition is provided on each of its sides with a plurality of vertical grooves or slots 58, Fig. 5, which are preferably, though-not necessarily, rectangular in cross section, and which extend in a direction perpendicularly to the axis of trion of the duct. The inner faces of the end members 84 and 5B are also provided with a plurality of vertical grooves 59 similar to the grooves 58.

The filter, provided where necessary with a suitable baiiie, or shield. not shown, to prevent sound from by-passing around it, or adapted to entirely fill the aperture of a conduit if used therein. is placed in the path of the sound wave with the axes of the ducts parallel to the direction of transmission of the sound wave.

In Fig. 3 is shown a filter of this same type adapted to a circular opening. In this form of construction the partitions 56a and theducts 51a differ from the corresponding parts of Fig. 4 in that in Fig. 3 the vertical dimensions of the ducts Us at any point conform to the height of the cross section at that point, the height of the ducts decreasing from the center line of the section outward toward either side.

I have discovered that such devices will act as effective acoustical band stop filters; and I have determined experimentally how. the characteristics of these filters depend on their constructional details and dimensions.

Before disclosing these results, it will be convenient to designate certain dimensional constants, or parameters, of the filter and also certain acoustical quantities by symbols, as follows:

I shall let H, L, and D represent the internal height, length, and width, respectively, of the ducts 51; T, the thickness of the vertical partitions or filter plates 58: d and w the depth and width, respectively, of the grooves or slots 58 in the faces of the filter plates: s the distance between centers of two adjacent grooves or slots II; n, the number of grooves or slots per face of the duct; 1. the lower cut-off frequency of the filter: F, the upper cut-off frequency; and b, the band width of the filter, i. e., FJ-

A typical transmission curve of a filter of the type shown in Figs. 4 and 5 is given in Fig. 8,

in which the ordinates represent the fraction of transmitted sound energy plotted to a logarithmic scale; and the abscissas represent the frequencies in cycles per second. The particular curve is that corresponding to a filter for which T=1.0 cm., D=8.0 mm., H=30 cm., L=53.2 mm., d=7 mm.. w=2 mm., and n=7.

It will be seen that the device suppresses very markedly all frequencies in a certain range and readily transmits frequencies outside of this range, i. e. acts as a band stop filter. Its eflectiveness is attested by the fact that the attenuation within the band exceeds 60 decibels, which means that within the band less than one millionth of the incident energy is transmitted.

Of primary importance in a filter of this type are the lower cutoff frequency f, the upper cutofl frequency F, and the band with b, that is F-f. I have found that. these quantities can be readily varied and adjusted to predetermined values by changing certain dimensional constants or paramaters of the filter, as follows:

Of marked effect on the cutoff frequencies I and F is the dimension D. The manner in which I and F vary with D, other factors being constant, is shown in Fig. 7'. It is seen that as D is increased, 1. e., as the filter plates 56 are spaced farther apart. the upper cutoff frequency decreases continuously, while the lower cutoff frequency increases rapidly at first, then more slowly, and eventually reaches a constant value, which does not change with further increase in the spacing of the plates. From this it will be seen that when the plates 56 are spaced relatively far apart the band width will be narrow; while when they are spaced relatively close together, the band width will be wide. It will also be seen that above a certain critical spacing the upper cutoff frequency alone varies with the spacing; while below this critical spacing both cutoff frequencies vary with the spacing.

Of marked effect on the lower cutoff frequency f, is the dimension d. The manner in which f varies with d, or more specifically l/d. is shown in Fig. 8. It is seen that as l/d increases. that is as the depth of the grooves 58 decreases, the lower cutoff frequency increases. If therefore. it is desired to extend the lower cutoff frequency to lower frequencies the slots 58 would be made deepen.

By varying the two dimensions D and d then, it is possible to vary greatly the characteristics of the filter, and adapt it to many specific purposes, in accordance with certain of the objects of this invention.

The other dimensional constants or parameters of the filter are of secondary importance: The width of the grooves is not material as long as w is less than d, and ordinarily will be from V3 to V: the value of d; the spacing s of the the steeper will be the cut-off, and the greater the attenuation within the band.

While I have shown grooves on both of the internal faces of the duct, it must be understood that a duct with grooves on only one surface will also act as a filter but will be only half as effective. Also while I have shown the grooves in opposite sides of the filter duct to be in line, it should be understood that this is not a necessary condition for an effective action of the filter but merely makes for convenience in manufacture.

It should be noted that my filter operates upon a basic principle entirely different from ordinary acoustical attenuation devices, such as sound labyrinths and sound absorbents. These latter devices operate upon the principle of acoustic resistance, as they absorb the sound energy in a manner analogous to the absorption of power by friction in a machine or to the absorption of electrical energy by resistance in an electrical circuit. My device, however, is a reactance type acoustic filter, employing a principle known as acoustic reactance in which the energy of one sound wave is used to neutralize the energy of a following sound wave without substantial absorption. 'I'o differentiate between these two basic types. it may be said that filters operating upon the principle of acoustic resistance dissipate the sound energy without afiecting the phase relationship of the sound waves. An acoustic reactance. however. throws the sound waves out of phase in such manner that the sound energy is no longer propagated in its original direction An acoustic reactance, however, does not absorb,

decrease or dissipate, in any manner, the incident sound energy. Sound labyrinths and sound absorbents both operate on the principle of acoustic resistance, for in a sound labyrinth the sound is forced to travel a relatively great distance before emission and utilizes the resistance of the air to absorb the sound energy.

Ordinary sound absorbents, such as glass wool, 1

or felt pads are well-known and require no explanation.

Referring to the drawings, sound waves entering the ducts 51 will enter the grooves 58 in the side walls of the duct and will be reflected back into the duct out of phase with the succeeding sound wave. The reflected sound wave provides destructive interference for the next succeeding incident sound wave and thus neutralizes the latters energy to a degree proportionate to the amount of the energy reflected by the groove. It is to be understood that a certain amount of sound will always be absorbed during its passage through the duct 5! and in the grooves 58 due to the viscosity of the air, but this absorption is largely negligible and occurs in all types of acoustical devices.

From the foregoing explanation it may be seen that I may, by adjustment of the dimensions of the groove, selectively remove sounds of any predetermined frequency and that within reasonable limits the reduction in sound energy effected by my filter will always be a' predetermined fraction of the incident sound energy.

It isnot to be understood, however. that my filter must be formed entirely of'sound reflective material as it may be seen that the base or bottom of the grooves is the principal sound refleeting portion and hence the emciency of the filter is controlled principally by the sound refiectivity of the exposed base wall of the grooves 58. It is generally conceded that all materials absorb sound to a certain degree and I may therefore vary the efficiency of my filter by varying the sound reflective properties of the material used to form the walls of the grooves.

I prefer to form the entire filter of a relatively dense sound reflective material such as metal, wood, glass or ceramic, but, if desired, the body of the filter may be formed of any other convenient material and the inner walls of the rooves, or only the bottom or base wall of the rooves, may be formed of a sound reflective material, such as those mentioned above. If the body of the filter is formed of a sound absorptive material, it is obvious that the filter will not be as sharply selective as it will be if the body of the filter is formed from a sound reflective material.

When a plane sound wave passes through the filter of Figs. 4 and 5, the disturbances at any instant at corresponding points such as P, P' (Fig. 5) of two adiacent slots on a line transversely of the filter are in phase, which means that the dividing portions a could be removed and the slots moved together and joined to form a common slot without impairing the action of the filter.

This leads to a form of construction shown in Fig. 9. Provided all dimensions, except T which is made equal to 2d, are retained, such a filter would duplicate very closely the characteristics of the filter of Figs. 4 and 5.

The filter of Fig. 9 is constructed conveniently by replacing each of the vertical slotted filter plates 58 of Fig. 5 by an assembly iii of vertical,

parallel laths 62 of rectangular cross-section, each lath being provided with apertures 53 for mounting on stringer bars 54 with interposed spacer washers 55, the stringer bars being threaded at their ends 65 to accommodate clamping nuts 51 and washers 58. The width of the transverse passages 59 may be varied as desired by varying the thickness of the spacer washers 65. The partitions may be spaced closer or farther apart by adjusting them in any suitable manner along the top and bottom plates 52 and 53. The individual assemblies or filter elements are mounted by securing them to the upper and lower frame members 52 and 53 by any suitable means. The end members 54 and 55 may remain as before.

A form of filter plate H which lends itself readily to production from .sheet metal is shown in Fig. 10. In this form, each of the vertical filter plates 55 of the filter of Fig. 5 is replaced by a sheet metal structure or filter element 1| bent into the shape shown to provide slots I2 alternately on opposite sides of the structure. The structures II can be mounted by securing them to uprights I! which are secured in turn to the frame members 52 and 53, or the structures H can be provided at their ends with suitable flanges (not shown) for securing them directly to the frame members 52 and 53.

A difilculty with the forms of filter described so far is that when it is desired to filter out very low frequencies, the depth d of the grooves has to be made large, with consequent increase in dimensions of the filter, which may then become b 1:2 discovered, however, that the working range of the filters described can be shifted to lower frequencies by providing the slots 58 (Fig. 12), with constricted entrance ports. These may take the form of parallel slits 15 in thin cover lates II, mounted one on each side of the vertical partitions 55, the slits being so arranged and spaced that there is one down the center line of each slot. as shown in Figs. 11 and 12.

In place of the slits, rows of circular apertures in cover plates 14a can also be used as in Fig. in order to shift the working range of the filter to lower frequencies.

By the addition of the entrance ports the original plain grooved filter plate is converted into a plate comprising a multiplicity of enlarged inner chambers 58 and communicating constricted entrance ports 15 and 15. In manufacture, of course, the cover plates 14 or Ha could be cast integrally with the partitions 55.

Since for low frequencies the inner chambers 58 will be of relatively large dimensions, for econcm; of space, a filter plate ll of the cross section shown in Fig. 14 is preferably used for low frequencies, and manufactured conveniently by casting or moulding a suitable material directly into this shape.

With sheet metal the form of filter plate 18 shown in Fig. 15 is preferably used. These plates may be made from a single sheet of metal as shown, and mounted by securing them to suitable supports 19.

Filters for low frequencies could also be economlcally built up of rows of units of the cross section shown in Fig. 16, each of the filter plates 55 of the original filter being replaced by a row of elements 50.

The elements 50 are straight, thin-walled conduits of rectangular cross section and identical dimensions, each of which carries a slot 58, parallel to the axis of the conduit, in one face. The slots are of identical dimensions and similarly placed in all the conduits.

The forms of filter so far described are primarily for use in cases where the incident wave front is plane or very nearly so. They will be efiective on nonplanar waves to a degree which depends on the extent of the departure of the wave front from the planar state. Where this departure is marked, the'forms of filter plates and filters now to b described will be more effective.

A form of filter plate for nonplanar waves is shown in Fig. 17. It comprises a plane plate 8! provided with a multiplicity of perforations 82 of rectangular cross-section extending partially or completely through the plate. The composite filter using these plates would be built up as the filter of Figs. 4 and 5 except that the plates 8| would replace the plates 55 and plane unperforated plates would replac the end members 54 and 55.

An alternative form of filter plate for nonplanar waves comprises a plane parallel plate 83 provided with a multiplicity of circular holes 84 extending partially or completely through the plate.

Another form of filter eil'ective on nonplanar waves is shown in Fig. 19. It comprises a thick plate 55 with a multiplicity of ducts 88 of square cross section extending therethrough, each duct being provided with slots or grooves 81, preferably of rectangular cross section, on all four of its inner faces 88. The plate 55 is placed in the path of the sound wave to be filtered.

A modified form of the invention, namely a band stop acoustic filter of variable band width, is shown in Figs. 20 and 21. It is clear from the graph of Fig. 'I that the band width of a filter of the type shown in Figs. 4 and 5 depends upon 75 the spacing of the filter plates 55, and therefore can be made variable, provided the filter is so constructed that the spacing of the filter plates is capable of variation. In the form of filter shown in Figs. 20 and 21 this is accomplished by interposing resilient spacer washers 9| between the separate filter plates 92 and also between the end members 93 and 94 and those of the filter plates 92 nearest to them. The filter plates 92 and end members 93 and 94, which otherwise may be identical with the filter plates 53 and end members 54 and 95, respectively, of Figs. 4 and 5, are provided at the four corners with circular holes 95 adapted to pass with a sliding fit round stringer bars 96.

The resilient spacer washers 9| (Fig. 23) which are preferably circular in form, all have identical dimensions and are cut from material of identical resilient properties, so that equal pressures applied to their faces will result in equal compressions for all of them. They are provided with circular holes 9'! somewhat larger in diameter than that of the stringer bar 98 to allow for change in diameter on compression.

The stringer bars 96 (Fig. 24) have a disc-like head 99 at one end; which head is provided with screw holes 99; while at the other end they are threaded to receive a specially shaped nut unit I00. The latter (Fig. 22) consists of a fiat calibrated disc I to which is attached rigidly a somewhat smaller nut shaped part I02. The disc IOI is provided with a scale I03 near its periphery. Nut and disc are preferably made in one piece.

The filter plates 92, 93 and 94 are mounted parallel by means of the stringer bars 98 and interposed spacer washers 9|, the heads 93 being rigidly secured to the end plate 93 by means of small screws II3 through the holes 99. After assembly, the plates 92, 93 and 94 are secured by tightening the nuts I00, a metallic washer I04 preferably being interposed. The nuts I00 are initially adjusted so that all the resilient washers 9| are equally compressed, and an index finger I05 is then placed in position at the zero line of the scale. Subsequently equal rotations of the nuts I00 will produce equal compressions of the spacer washers 9| and result in equal variation of the spacing of the filter plates 92, 93 and 94.

The resulting assembly I06 is closed at top and bottom by means of the upper and lower cover plates I01 and I08, which are provided with retaining fianges I09. The end plate 93 is secured rigidly to the cover plates by screws H4. The edges IIO of the filter plates 92, 93 and 94 and the inner surfaces III of the plates I01 and I08 as well as of the flanges I09 are carefully fitted, so that the plates 92 and the plate 94 may slide freely on the retaining plates I 01 and I08. The assembly is provided on the side'of the plate 94 with a cover plate II2 of sufiicient width to allow the assembly I 09 to contract laterally without allowing the bypassing of sound around the side of the filter.

It is also possible to construct a filter so that the lower cutofi frequencies may be varied within prescribed limits. This may be accomplished by employing a filter of the type shown in Fig. 9, but separating the laths 92 by small rectangular rubber strips extending from top to bottom, along the center line of the lath assembly 9|. With such an arrangement the depth of the transverse grooves may be varied merely by tightening the nuts 01, causing the strips to be compressed to effectively reduce the depth of the grooves.

III

Another modification of my invention as shown In Fig. 25, is concerned with the sup pression of noises in the delivery of air from high pressure ventilating conduits to rooms and oifices. In order to maintain the pressure in a high pressure ventilating conduit I2I supplying a room I20, it is necessary to pass the air through a number of constricted openings, such as for example, the slits I22 of a louver valve I23. However, the passage of air under pressure through such constrictions is attended by the production of considerable noise, which may be objectionable to the occupants of the room or oilice which the conduit supplies. In order to avoid this difiiculty, in one form of my invention, the constricted louver valve I23, leading from a high pressure ventilating conduit I2I is followed by an expansion chamber I24, and be- -fore entrance into the room the air rushing through the slits I22 is passed through a filter I20 of the type of Figs. 4 and 6 with a cross-section relatively large to that of the conduit I2l and with a relatively large number of filter ducts 51, so that it is adapted to filter out noises generated by the rush of air through the slits of the louver valve and yet offer a minimum of impedance to the fiow of air. It is to be understood of course that this form of the invention is not limited to louver valves with narrow slits but can be used with any equivalent device having a multiplicity of constricted openings through which air under pressure is allowed to stream. The louver valve may be replaced, for example, by a plate with a series of narrow slits, or a plate with a multiplicity of small apertures, etc.

In the improved form of the invention shown in Fig. 26, the louver valve of Fig. 14 is replaced by an acoustic filter of the type of Figs. 4 and 5. In this arrangement, a high pressure main I3I delivers air to a room through an acoustic filter I32, an expansion chamber I33, and a second acoustic filter I34. The acoustic filter I32 is con struoted with relatively few filter ducts in order to maintain the pressure within the main I3I, while the filter I34 is constructed with a relatively large number of filter ducts. The filter I32 serves to filter out any fan or motor noises attempting to pass into the room from the high pressure main as well as to maintain the pressure in the main; while the filter I34 serves in turn to filter out any noises created by the air streaming from the high pressure main through the filter I32 into the expansion chamber.

In most air conditioning systems, because of the fact that a low speed blower is necessary to reduce objectionable noises to a minimum, the distribution of air in the duct system is accomplished by deflectors which depend upon air velocity for their effectiveness. As a result it is virtually impossible to have branch ducts connect with the main duct at angles of degrees or less with respect to the direction or air fiow. By using a high speed fan, however, in conjunction with sound filters of type disclosed in this application, and creating pressure within the main duct in order to fiect distribution of air to the branch ducts, it is possible to have the branch ducts connect with the main duct at any angle that is most convenient for the particular installation.

I claim;

1. An acoustic filter comprising a pair of filter elements mounted in spaced relation, each of said filter elements comprising a sequence of straight bars of rectangular cross section and identical dimensions, means for mounting said bars in line, and means for holding said bars in spaced relation.

2. An acoustic filter comprising a pair of plane plates mounted parallel to each other and in spaced relation, at least one of said plates being provided internally with a plurality of parallel, straight and relatively elongated chambers of constant cross section, and a plurality of relatively constricted entrance ports leading from the surface of said plate to said chambers.

3. An acoustic filter comprising a pair of plane plated mounted parallel to each other and in spaced relation, at least one of said plates being provided internally with a plurality of parallel, straight and relatively elongated chambers of constant and identical cross section, and a plurality of relatively constricted entrance ports leading from the surface of said plate to said chambers.

4. An acoustic filter comprising a duct of rectangular cross section, at least one internal face of which is provided with a plurality of straight slots of rectangular'cross section extending lengthwise in a direction perpendicular to the axis of the duct, a plurality of elongated chambers of rectangular cross section extending lengthwise in a direction parallel to said slots and so located as to communicate one with each slot, the cross section of said chambers being enlarged relatively to the width of the slot.

5. An acoustic filter comprising a duct of rectangular and relatively elongated rectangular cross section, a plurality of relatively elongated chambers of rectangular cross section ranged laterally of said duct. and a plurality of relatively'constricted entrance passages connecting sound filter of the resctance type comprising a body having a plurality of openings for the transmission of air and grooves in the walls of the openings extending lengthwise in a direction which is perpendicular to the axis of transmission of the air, and having sound reflecting surfaces, said sound filter being adapted to prevent the passage of only a selected band of sound waves.

'7. A selective band type sound filter in the form of a duct including a rigid element supported in spaced relationship to a wall of the duct the surface of the element facing the duct wall having formed therein an elongated groove having a sound reflecting bottom wall spaced inwardly from the surface of the element a distance substantially equal to one-fourth the wave length of the sound of lowest frequency to be attenuated, the groove being disposed approximately transverse to the axis of sound propagation through the duct.

8. A selective band type acoustic filter in the form of a duct including a smooth and rigid late of high apparent density facing one of the inner surfaces of the duct, the plate having a plurality of parallel right angle bends forming a plurality of elongated grooves of regular crosssection alternately on opposite faces of the plate whereby sounds having wave lengths substantial- 3' equal to four times the depth of the grooves gillt be attenuated while passing through the 9. A selective band type acoustic filter including a plurality of substantially rigid elements pported in spaced relationship to each other defining passageways, the opposing faces of each element having formed therein a plurality of elongated grooves of substantially constant crosssection, each of said grooves being transverse to the axis of transmission of the passageways, each groove having substantially smooth sound refiecting walls, the depth of the grooves being not greater than one-fourth the wave length of the lowest sound frequency in the attenuated band and means for varying the effective width of the passageways between the elements.

10. A selective band type acoustic filter in the form of a duct including a rigid member having an exposed surface within the duct, said surface having formed therein a plurality of elongated grooves having exposed relatively smooth sound reflective walls, said grooves being of constant cross-section and having their major axes transverse to the direction of sound transmission through the duct, the filter being characterized by a direct variation between the lower band outof! limit and the depth of the groove.

11. A selective band type acoustic filter including a plurality of substantially rigid elements supported in spaced relationship to each other defining passageways, the opposing faces-of each element having formed therein a plurality of elongated grooves of substantially constant crosssection, each of said grooves being transverse to the axis of transmission of the passageways, each groove having substantially smooth sound reflecting walls, the depth of the grooves being not greater than one-fourth the wave length of the lowest sound frequency in the attenuated band and means including resilient members interposed between the rigid elements for varying the effective width of the passageways between the elements.

12. A selective band type sound filter in the form' of a duct having a rigid element supported in spaced relationship to a wall of the duct at least the surface of the element facing the .duct

wall having formed therein a plurality of elongated grooves having sound reflecting bottom walls spaced inwardly from the surface of the element a distance substantially equal to onefourth the wave length of the sound of lowest frequency to be attenuated. each of said grooves being of substantially constant cross-section and disposed approximately transverse to the axis of sound propagation through the duct.

18. A selective band type acoustic filter in the form of a duct including a plurality of rigid elements supported in spaced relationship to a wall of the duct and to each other the faces of each element having formed therein a plurality of elongated grooves of substantially constant crosssection, each groove having a substantially smooth sound reflecting bottom wall spaced inwardly from the surface of the element a distance not greater than one-fourth the wave length of the sound of lowest frequency to be attenuated by the filter, each of said grooves being disposed transversely to the axis of sound propagstion to the duct.

HAROLD K. SCHILLING.

7 CERTIFICATE OF oonnsc'non. Patent No. 2,299,112. October 20, 191;.2.

HAROLD x. scHILLINo.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 6, first column, line 11;, claim}, for "plated" read "plates"; line 11,7, claim 6,

strike out "of the reactance type and insert the came before "in the" in line 1 6, same claim; and that the said Letters Patent should be read with this correction therein that the samev may conform'to the-record of the case in the Patent Office. I Signed and'sealed th'is 2mm day of November, A. n. 19).;2.

Henry Van Arsdale; (Seal) Acting Commissioner of Patents.

Patent No. 2,299,112.

ER'l'IFICATE OF CORRECTION.

.October 20; 191m;

"HAROLD K. SCHILLING.

It ,is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 6, first column, line 11+, claimfi, for "plated" read --plates--;- line 14.7, claim 6,

strike out "of the reactance type" and insert the same before "in the" in line 14.6, same claim; and that the said Letters Patent should be read with this correction therein that the same. may conformto th9-recprd of the case in the Patent Office. I

, si ned and' sealed this alum day of November, A. D. 191.;2.

Henry Van Arsdale',

(Se'al)- Acting Conmissioner of Patents. 

